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A car traveling at a certain constant speed takes 2 seconds

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Re: A car traveling at a certain constant speed takes 2 seconds  [#permalink]

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New post 09 Nov 2018, 10:58
The speed is mentioned in kilometers per hour but the difference is mentioned in seconds. So we convert speed into kms per second by dividing 75 by 3600

Since Speed = distance/time or time = distance/speed (distance is 1 kms) we have time = 1/(75/3600) or t = 48 seconds.

Since the car takes 2 seconds longer to travel 1 kilometer than it would take to travel 1 kilometer at 75 kilometers per hour. we have t = 48 + 2 = 50 seconds.

Now speed = distance/ time. First we convert time of 50 seconds in hours by divding by 3600 to get 50/3600

Therefore speed = 1/(50/3600) = 3600/50 = 72 (Ans B)
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Re: A car traveling at a certain constant speed takes 2 seconds  [#permalink]

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New post 09 Nov 2018, 11:27
RadhaKrishnan wrote:
A car traveling at a certain constant speed takes 2 seconds longer to travel 1 kilometer than it would take to travel 1 kilometer at 75 kilometers per hour. At what speed, in kilometers per hour, is the car traveling?

A. 71.5
B. 72
C. 72.5
D. 73
E. 73.5


Extra time taken = 2 sec = 2/3600 hour = 1/1800 hour

Time taken to travel 1 km at 75 km/hr = 1/75 hour

Time taken for car = 1/75 +1/1800 = (24 + 1)/1800 = 25/1800 = 1/72 hour

Speed = distance/time = 1/(1/72) = 72 km/hr

Answer choice B

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A car traveling at a certain constant speed takes 2 seconds  [#permalink]

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New post 27 Mar 2019, 22:42
Time taken to travel 1km at 75km/h = 1/75 h = 3600/75 s

It takes 2 s longer -> 3600/75 + 150/75 = 3750/75 = 50s

Speed = D/T -> 1000m/50s = 20m/s

To convert from km/h to m/s -> multiply by 5/18. For ex 18km/h = 18*5/18 = 5m/s
Similarly to convert from m/s to km/h -> multiply by 18/5. For ex 5m/s = 5*18/5 = 18km/h

5/18 is just a simplified form of 1000/3600 and vice versa. I remember this method from my middle school days and it has definitely simplified things for me.

So back to 20m/s = 20*18/5 km/h = 4*18 = 72km/h (B)
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A car traveling at a certain constant speed takes 2 seconds  [#permalink]

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New post 28 Mar 2019, 03:06
Suppose the car has travelled with the speed of 75km/hr for 1 hour.
So, t1=60 mins; d1=75 kms; s1= 75km/hr

Now with the new speed the car takes 2 seconds more to travel 1 km. So the car will take 150 seconds more to travel 75 kms, that is, 2.5 mins more. Therefore, t2=60+2.5=62.5

We are comparing for the same 75 km run therefore d1=d1,
s1*t1=s2*t2
75*60=s2*62.5
s2=72km/hr

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A car traveling at a certain constant speed takes 2 seconds   [#permalink] 28 Mar 2019, 03:06

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